1. Field of the Invention
This invention relates generally to high resolution radar systems including apparatus for generating a synthetic antenna and more particularly to a method and means for providing in-flight, high resolution radar mapping under conditions of relaxed aircraft maneuvers.
2. Description of the Prior Art
In present day side-look synthetic aperture radars which are utilized for high resolution radar mapping, such systems are generally constrained to straight and level flight paths, or at most very limited maneuvers during the mapping run. Such flight courses, however, are hazardous in a hostile environment and conversion maneuvers for ground strike are limited. An extremely narrow flight corridor is thus required for very high resolution mapping even when a well known first order motion compensation procedure is applied. Such a teaching is found in Synthetic Aperture Radar Systems, R. O. Harger, Academic Press, 1970, pages 146-155, inclusive.
Generally, such first order motion compensation methods adjust the phase of received radar return signals so as to cancel the phase shift caused by small flight paths deviations from a straight line thus generating a uniform holographic signal format which is required for optical correlation. This type of motion compensation however is basically incompatible with the dynamically changing geometry associated with a tactical aircraft. Other problems also arise with this method of compensation when high resolution operation is desired under high aircraft acceleration. Excessive acceleration along the target line of sight occurs when the range curvature (quadratic component) during a coherent integration period exceeds the design range resolution, i.e. a point target crosses from one range gate to another even though the range gates are accurately tracking the velocity (linear component). An excessive acceleration component perpendicular to the target line of sight but in the aircraft velocity line of sight plane projects intolerable acceleration errors at azimuth angles away from the azimuth boresight even though perfect compensation is applied at all points along the intersection of the azimuth boresight with the ground. Thus it is fundamentally impossible to cancel these errors with a single correction as the complex radar return signal arrives. Uncorrected, these problems cause target decorrelation resulting in an intolerable amount of map smear.